The invention relates generally to the area of gas sensing. More specifically, the invention relates to the sensing of NOx gas.
Environmental considerations are the primary motivating factors to develop NOx gas sensors. NOx emissions react with gases such as SOx, CO and moisture (water vapor) in the air to produce smog and acid rain. One of the major sources of NOx emissions is automobile exhaust.
The European Euro VI emission standards for light commercial vehicles (category N1-I, N1-II and N1-III), to be implemented by September 2015, require NOx emission levels below 0.5 gm/hp-hr. This typically translates to less than 50 ppm of NOx tail pipe emissions. Development of cost-effective gas sensors that can give reliable readout at such low concentration levels of analyte, and which can deliver robust performance even in harsh environments, is one of the major challenges facing present day emissions monitoring technology.
The current paradigm in improving the efficiency of internal combustion engines utilizes the technology of lean burn, whereby very high air:fuel ratios (˜102:1), as compared to conventional stoichiometric ratio (typically ˜20:1), are used. While the lean burn technology improves the efficiency of the engine, it also results in higher NOx emissions.
Any emissions control scheme that adversely impacts or limits efficiency will not be commercially viable. This necessitates real time monitoring of NOx emission levels and use of this information to dynamically control engine operating parameters (such as compression ratio etc) and exhaust after-treatment systems (such as catalytic filters etc) to achieve enhanced engine efficiency and enhanced emissions control respectively.
One of the current NOx gas sensing technologies in the market employs yttria stabilized zirconia (YSZ) based gas sensors. The gas sensors are essentially a multi-chamber electrochemical cell measuring the oxygen changes as a result of NOx decomposition. Such technology requires catalysts such as Pt. However, the performance of the catalyst degrades upon exposure to SOx and water vapor, as are commonly present in the exhaust from automobiles. This is one of the factors contributing to lowering the working life of such gas sensors. Further, the relatively intricate design of these gas sensors makes them expensive to replace on a regular basis.
Another current gas sensing technology in the market employs semiconductor gas sensors. As with any technology, this technology presents situation specific disadvantages and advantages. For example, gas emissions monitoring applications often require quantitative estimation of a particular or few gas species (e.g., NOx) in a multiple gas species environment. These gas sensors however, are sensitive to a broad range of gases, and therefore are of limited utility in such applications. Furthermore, these gas sensors are prone to long term instability because of their polycrystalline nature. On the other hand, this technology has the advantages of being solid-state, such as rigid construction and compact size. Further, the technology is amenable to readout using simple electronics thereby reducing cost of system manufacture, operation, maintenance and replacement. In addition, semiconductor gas sensors admit wide range of response tunability via introduction of suitable dopants, control of morphology of gas sensing surface, control of gas sensor operating parameters, amongst other controllable factors.
A gas sensor that is semiconductor based, can make quantitative estimation of NOx gas even at low concentration levels, and have a long working life, would therefore, be highly desirable.